The 3rd Generation Partnership Project (3GPP) stipulates that user equipment (UE) needs to satisfy a state of being registered (a registration state) with an IP-CAN in order that the UE can use a service through an IP Multimedia core network Subsystem (IMS). As for the registration state of the UE, a session control apparatus, or more specifically, a Proxy-Call Session Control Function (P-CSCF) located at an interface with an IP access network (IP-CAN) holds a subscriber profile of the UE (an identifier, subscription information, and the like of the UE).
Whether or not the P-CSCF is operating normally is diagnosed by a Keep-alive function (a Heartbeat function) executed regularly between a Packet Data Network Gateway (PGW) and the P-CSCF. There has been defined a method in which, if the P-CSCF is diagnosed as not operating normally, the PGW prompts UE, which is registered with the P-CSCF, to perform re-registration via the network (see Non-patent Document 1, for example).